On 11/23/2009 03:49 PM, Alan Stern wrote:
On Mon, 23 Nov 2009, Ric Wheeler wrote:
What we need is to pass down cache flush commands (ATA_CACHE_FLUSH_EXT is what
flushed the cache for ATA/S-ATA devices). Even with a command-queue length of 1,
the write cache a USB connected s-ata drive would still loose data on power off
without this kind of support.
You can't send ATA commands directly over a USB connection without
using a passthru protocol of some sort. Some of the USB interfaces in
various drive enclosures support such a protocol but a lot of them
don't.
Of course, you _can_ send SCSI's SYNCHRONIZE CACHE command. What
ATA/SATA command (if any) the drive's USB interface will translate it
into is known only to the interface's manufacturer. Nevertheless,
that's what we've been depending on because that's what the sd driver
uses.
Alan
That makes sense - so the usb storage stack does the right thing, what happens
is up to the SCSI -> ATA translation in the target device.
If I still had occasional access to analyzers, we could always test & verify
that some of the common enclosures work correctly but I have unfortunately lost
my toys :-)
Thanks!
Ric
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